General Single Term Keyword Drawback
When picking keyword to implement in your keyword tags for your website, please remember this rule! DO NOT USE SINGLE TERM UNLESS THE TERM ITSELF IS SPECIFIC.
For example, if you pick "MLM" term as the keyword tags in your webpage, I think most likely you don't really know what the target you are aiming. Why? "MLM" is a general term. Although this keyword has high searches, most people when they search for this "MLM" term probably still searching for something. Instead of aiming for the "MLM" term, it could have been wise for you to pick terms like "Amway", "Avon", "eCosway", etc.
Wouldn't you agree with me that people searching for the specific terms (Amway, Avon, eCosway, etc.) like the above surely they're interested in a specific multi level marketing? They are interested on it and if they don't, they won't key in the MLM company names. Agree?
What is the drawback when you use a general single term keyword?
1. Untargeted traffic
Think now ... if A key in "MLM" in Google and B key in "Amway", who do you think are more likely targeted?
2. Low Conversion Rate
If untargeted traffic clicking on your sales webpage, what you will get? You will get prospects which are just playing around your webpage (because they're unsure of what they searching for) and then click quit. Correct?
I think it doesn't make sense to use a general single term keyword for your webpage unless they keyword itself is very specific, but frankly single term keywords are not worth the effort.
Further recommended resources Keyword Search @ Hubpages which provides informative content on keyword research for SEO.